When boat QNG 90675 limped into the Vietnamese port at Quang Ngai on Dec. 12, it showed signs of a confrontation at sea with a Chinese coast guard ship. According to local officials, the coast guard boarded the boat, forced the crew of seven to stand with their hands behind their heads, and confiscated their catch and fishing gear, before cutting the boat’s nets and casting them into the sea.

The boat had been fishing in the Paracel Islands in the South China Sea, as Vietnamese fishermen have for centuries, inside what Hanoi claims as the country’s exclusive economic zone in what it calls the East Sea. With coastal areas of Vietnam overfished, the fishing grounds around the islands abound in species such as tuna and flying fish.

But the Paracels also lie inside China’s self-declared maritime boundary— the “nine-dash line”— the swath of the sea within which Beijing claims exclusive rights. It does not hesitate to demonstrate this against Vietnam’s fishermen, sometimes in crude terms.

The manager of the fleet that includes QNG 90675 says the Chinese vessel accosted the boat and officers jumped on board, saying they were fishing in Chinese waters. “The area where we fish is Vietnamese water,” says Duong Van Rin, who manages 20 boats. “We aren’t violating any country’s waters.”

Vietnamese industry officials and fishermen say confrontations between Chinese coast guard or civilian boats — referred to by state media in Hanoi as “alien vessels” — and Vietnamese craft are frequent, and have in several cases seen boats sunk, fishermen injured and equipment or catches stolen.

The fights over fishing represent a little-reported economic and environmental facet of the clash between China and its Southeast Asian neighbors in one of the world’s hottest geopolitical trouble spots, as well as a potential source of a broader U.S.-China conflict. Fishermen, as academic Gregory Poling of the Center for Strategic and International Studies (CSIS) wrote in a research paper this month, “serve on the front lines of this contest” as they fight for their livelihoods against bigger, better-equipped Chinese coast guard and fishing boats.